A thermal processing chamber as used herein refers to a device that rapidly heats objects, such as semiconductor wafers. Such devices typically include a substrate holder for holding one or more semiconductor wafers and a light source that emits light energy for heating the wafers. During heat treatment, the semiconductor wafers are heated under controlled conditions according to a preset temperature regime. During heating, various processes can be carried out within the thermal processing chamber, such as rapid thermal oxidation, nitridation, annealing, silicidation, sintering and metallization.
Many semiconductor heating processes require a wafer to be heated to high temperatures so that the various chemical and physical transformations can take place as the wafer is fabricated into a device. During rapid thermal processing, for instance, semiconductor wafers are typically heated by an array of lights to temperatures from about 300.degree. C. to about 1,200.degree. C., for times which are typically less than a few minutes. During these processes, one main goal is to heat the wafers as uniformly as possible.
During the rapid thermal processing of a semiconductor wafer, it is desirable to monitor and control the wafer temperature. In particular, for all of the high temperature wafer processes of current and foreseeable interest, it is important that the true temperature of the wafer be determined with high accuracy, repeatability and speed. The ability to accurately measure the temperature of a wafer has a direct payoff in the quality and size of the manufactured integrated circuit. For instance, the smallest feature size required for a given semiconductor device limits the computing speed of the finished microchip. The feature size in turn is linked to the ability to measure and control the temperature of the device during processing.
One of the most significant challenges in wafer heating systems is the ability to accurately measure the temperature of substrates during the heating process. In the past, various means and devices for measuring the temperature of substrates in thermal processing chambers have been developed. Such devices include, for instance, pyrometers, thermocouples that directly contact the substrate or that are placed adjacent to the substrate, and the use of laser interference.
Currently, of the above temperature sensing devices, pyrometers are generally preferred for use in thermal processing chambers. Pyrometers are capable of measuring the temperature of a substrate, such as a semiconductor wafer, without contacting the substrate. For example, one advantage of using a non-contact pyrometer is that the substrate can be spun slowly during the heating process which promotes uniform temperature distribution and promotes more uniform contact between any gases flowing through the chamber and the substrate. Besides being able to rotate the wafers, another advantage to using pyrometers is that, since no temperature gauges need to be attached to the substrate, the substrate can be processed much more quickly saving precious time during semiconductor fabrication.
In order to use pyrometers in a thermal processing chamber, the pyrometers generally need to be calibrated. Consequently, various calibration procedures currently exist in order to align the temperature readings of the pyrometers with some absolute and accurate temperature reference. The current state of the art and the most widely used method to calibrate temperature devices, such as pyrometers, in thermal processing chambers are to place in the chambers a semiconductor wafer having a thermocouple embedded in the wafer. The temperature measurements taken from the thermocouple are compared with the temperature readings received from the temperature measuring devices and any discrepancy is calibrated out.
Another method that has been used in the past to calibrate temperature sensing devices, such as pyrometers, contained within thermal processing chambers is to heat a substrate within the chamber that undergoes a chemical or physical transformation when heated to a particular temperature. By observing or measuring the chemical or physical transformation that occurs, the temperature to which the substrate was heated can be accurately determined which can then be used to calibrate other temperature sensing devices contained within the chamber. For example, in one embodiment, silicon oxidation can be carried out within the chamber by heating a silicon substrate. The amount or extent of oxidation that occurs when the substrate is heated indicates the temperature to which the substrate was exposed. Besides silicon oxidation, other calibration methods include ion implant activation, such as As+ implant or BF.sub.2 + implant, and silicidation of refractory metals, such as titanium and cobalt.
Although the above methods are well suited to calibrating temperature measuring devices, such as pyrometers, the above methods require a substantial amount of time in order to calibrate the instruments. As such, a need currently exists for a method of calibrating pyrometers in thermal processing chambers very rapidly without creating a substantial amount of down time. In particular, a need exists for a method of calibrating pyrometers in thermal processing chambers without having to open the chamber, in order to maintain chamber integrity and purity. A need also exists for a simple method for calibrating pyrometers in thermal processing chambers that can be used routinely as a regular check to verify that the optical pyrometry system is properly functioning.